1. Technical Field
The technical field includes all electrical heating and safety systems, particularly heating pads and electric blankets that include safety systems for overheat protection under abnormal use conditions.
2. Description of the Prior Art
Electric heating pads are put through numerous abnormal conditions by consumers. To ensure their safety, an overheat safety protection element is commonly included. It is not uncommon for a consumer to unintentionally abuse the product by bunching, twisting and folding the product. While heating pads or electric blankets need to meet consumer demands with faster preheats, higher temperatures and improved comfort, they also need to meet safety requirements with safety circuits and smart wire construction.
Modern flexible heating wire, such as used in electric blankets and heating pads, senses the wire temperature and provides a feedback signal to the control to control both the temperature and safety of the product. The present inventor has several inventions in the area of temperature control and safety of flexible heating wire that use the characteristics of the wire in combination with an electronic control circuit to accomplish temperature control and safety. Weiss U.S. Pat. No. 5,861,610 discloses a heater wire for use in a heating pad and electric blanket, which heater wire includes a sensor wire. An electronic control senses the resistance change with temperature of the sensor wire, and the electronic control also looks for a voltage indicating a meltdown of the inner insulation. Keane U.S. Pat. No. 6,222,162 discloses an electric blanket having a heater wire, and a control that measures the resistance change of the heater wire using a series resistor without a separate conductor. Though the method disclosed in the aforementioned Keane patent can sense the average temperature of the wire, it is limited because hot spots due to bunching or abnormal folding are not sensed. Gerrard U.S. Pat. No. 6,310,332 discloses a heating blanket which uses a combination of a low melt NTC (negative temperature coefficient) layer and a series resistor to control and sense hot spots. The heater wire is powered under one-half (½) cycles, and the sensor wire looks for current in the other half cycle to sense a wire hot spot. Weiss U.S. Pat. No. 7,180,037 discloses a heater wire and control for use in a heating pad and electric blanket that use a separate sensor wire and an NTC layer between the sensor wire and heater wire that conducts current when the first insulation layer becomes hot and also monitors the temperature of the heater wire itself. Temperature sensing of both the NTC layer and the heater wire is accomplished without a series resistor by a phase shift measurement. Systems that include an NTC (negative temperature coefficient) polymer as the insulator for both the function of the circuit and program (software) involved in the safety aspects of the control utilize analog circuits and a microcontroller. Multiple critical components are often identified whose tolerance and manufacturer supply are specified. The failure mode analysis is based on the accumulated failure rates of these multiple critical components, including the microprocessor and solid state switches, such as triacs. The more components that contribute to the safety circuit result in a shorter time between failures. The ingenious circuits that have a reduced number of critical components and also provide improved wire fault detection have led to the success of “smart wire” systems. The disclosures set forth in each of the above-identified patents are incorporated herein by reference.
The extensive approval process in combination with diverse product offering and a short technology life cycle has hampered the cost effectiveness of introducing new technology, i.e., a heating pad or electric blanket having a different shape and wattage approved on an individual model basis is expensive and the approval process is lengthy. Layers of redundant safety systems come at a price, although the reliance on sophisticated electronics is a safety improvement over the traditional mechanical thermostat systems. The consumer is not always willing to pay additional for features that are transparent, resulting in the less reliable mechanical temperature control products that are still evident in today's lowest cost heating pads.